1. Field of Invention
This invention relates generally to hybrid vehicles and more particularly to starting an engine in a hybrid vehicle.
2. Description of Related Art
Hybrid electric vehicles having an engine (such as an internal combustion engine) and an electric motor, for providing power to the vehicle have become a viable alternative to conventional internal combustion engine vehicles. While such vehicles may require more complex power transmission components, this complexity is offset by improved fuel consumption and a corresponding reduction in pollutant emissions from the engine.
Given the present climate of higher prices for fossil fuels, there is a corresponding desire to further reduce fuel consumption costs when operating hybrid vehicles. Hybrid electrical vehicles reduce fuel consumption by apportioning the power required to operate the vehicle between the engine and electric motor, to cause these components to operate at efficient operating points. For example, when moving slowly or when starting off from a stationary position, the electric motor may be considerably more efficient than the engine and in this case most of the power may be supplied by the motor. Engines generally operate more efficiently at higher rotational speeds and accordingly, at higher vehicle velocity a greater proportion of power may be supplied by the engine. When moving slowly, it may thus be desirable to supply all of the necessary operating torque from the motor, while the engine remains stopped, thus reducing fuel consumption and emissions.
In some hybrid vehicles, the electric motor is coupled to the engine and may be used to start the engine, thus eliminating the need for a separate electric starter motor, as found in conventional internal combustion engine vehicles. When the vehicle operates under conditions that require torque to be supplied from the engine, the motor may be mechanically coupled to the engine to provide a starting torque thereto. The diversion of torque from the vehicle wheels to the engine while starting the engine may result in undesirable acceleration changes.
There remains a need for improved methods and apparatus for starting engines in hybrid vehicles.